The invention relates generally to cryogenic liquid storage tanks, and, more particularly, to an improved storage tank for such liquids that features an improved ullage tank arrangement.
Cryogenic liquids are liquified gases that have boiling points generally below -150.degree. F. at atmospheric pressure. Examples of cryogenic liquids include liquid natural gas (LNG), nitrogen, oxygen, carbon dioxide, methane and hydrogen.
Cryogenic liquids are usually stored in thermally insulated tanks that consist of an inner storage vessel mounted within an outer shell. The space between the inner vessel and outer shell is typically filled with insulation material and a vacuum may be drawn therein. Such an arrangement minimizes the transfer of heat from the ambient environment to the cryogenic liquid stored within the tank so that vaporization is minimized.
A cryogenic storage tank, no matter how well insulated, however, will always suffer from heat leakage between the ambient environment and the liquid cryogen. As a result, over time, the cryogenic liquid will warm. This causes the cryogenic liquid to expand so as to increase the pressure within the tank. As more time passes, the pressure in the tank will continue to increase. When the pressure reaches a critical level, it becomes necessary to vent the tank and release some of the vapor.
Single line or hose filling of such tanks is accomplished by spraying subcooled cryogenic liquid, that is, cryogenic liquid that is at a temperature and pressure below its vaporization point, into the top of the tank. This allows the vapor present in the tank to be collapsed and recondensed into liquid. As a result, venting is not required during filling and product losses associated therewith are avoided. In addition, metering is greatly simplified since, with the absence of venting, there is no outflow to subtract from the product delivered.
A problem associated with single line filling, however, is that it allows a tank to be filled almost 100% full of liquid. This severely reduces the hold time of the tank in that there is no room to accommodate the expansion of the cryogen as it warms over time.
Prior solutions to this problem have included mechanical and electronic liquid level sensing devices that stop the flow of cryogen into the tank prior to it becoming 100% full. Such devices, however, utilize either moving parts inside of the tank, which are prone to freezing, and/or external electronic connections, which are prone to damage or corrosion. In response to the shortcomings of such liquid level sensing shutoffs, cryogenic liquid storage tanks featuring ullage tanks have been developed.
U.S. Pat. No. 5,404,918 to Gustafson, assigned to the present assignee, discloses a cryogenic storage tank that features a main storage tank with a smaller ullage tank positioned therein. The tanks communicate through a relatively small passage in the bottom of the ullage tank. The passage has a flow rate capacity up to 30% of the main tank fill line. Because the fill line is significantly larger than the passage, the main tank will fill with liquid while the ullage tank remains substantially empty except for cryogenic vapor. When the main tank becomes full, a sharp pressure rise occurs due to the increased flow resistance of the passage. As a result, the flow into the main tank will decrease dramatically. This is detected by an external flow monitoring device and the fill operation is stopped. The vapor space left in the ullage tank allows space for liquid expansion due to heat leakage.
The liquid cryogen in the main tank cools the walls of the ullage tank so that the vapor therein is cooled to the temperature of the liquid in the main tank. As a result, a portion of the vapor in the ullage tank condenses so that the pressure therein decreases. The liquid in the ullage tank also reaches the same temperature as the liquid in the main tank. The minor pressure difference between the head spaces of the main tank and the ullage tank causes liquid to flow into the latter. As a result the liquid levels and the head space pressures in the main tank and the ullage tank will tend to equalize. When product is withdrawn from the main tank, liquid will flow from the ullage tank so that the liquid levels and head pressures in the two tanks once again equalize.
While the cryogenic liquid storage tank of the '918 patent provides a dramatic improvement in hold time over a tank that is 100% full of liquid cryogen, the vapor space provided for expansion is not constant. For example, a half full tank that has been standing idle for a period of time will generally have a half full ullage tank. If the main tank is then refilled, only half of the ullage tank space is available for its intended purpose.
A different system for controlling liquid levels is presented by U.S. Pat. No. 5,411,374 to Gram. The '374 patent discloses a system whereby a secondary tank is drained prior to the withdrawal of liquid from the main tank. The secondary tank communicates with at least two lines. One line allows liquid from the main tank to expand into the secondary tank. The second line allows liquid to be withdrawn from this tank. The withdrawal lines from the main tank and the secondary tank are externally connected to a control system that determines when to withdraw liquid from each tank to empty the secondary tank first and control the pressure in the main tank. While the system of the '374 patent is effective, it is difficult to construct and requires the use of external controls. These external controls add cost and maintenance requirements to the system.
Accordingly, it is an object of the present invention to provide an improved cryogenic liquid storage tank that permits control of the ullage tank liquid level.
It is another object of the present invention to provide an improved cryogenic liquid storage tank that permits control of the ullage tank level through thermodynamic principles instead of mechanical or electrical controls.